RT9168-22CSH datasheet(7/13 Pages) ETC

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RT9168/A

DS9168/A-03 May 2001

www.richtek-ic.com.tw

Applications Guides

Capacitor Selection and Regulator Stability

Like any low-dropout regulator, the external capacitors

used with the RT9168/A must be carefully selected for

regulator stability and performance.

Using a capacitor whose value is > 1

µF on the

RT9168/A input and the amount of capacitance can be

increased without limit. The input capacitor must be

located not more than 0.5" from the input pin of the IC

and returned to a clean analog ground. Any good

quality ceramic or tantalum can be used for this

capacitor. The capacitor with larger value and lower

ESR (equivalent series resistance) provides better

PSRR and line-transient response.

The output capacitor must meet both requirements for

minimum amount of capacitance and ESR in all LDO

applications. The RT9168/A is designed specifically to

work with low ESR ceramic output capacitor in space-

saving and performance consideration. Using a

ceramic capacitor whose value is at least 1

µF with

ESR is > 5m

Ω on the RT9168/A output ensures

stability. The RT9168/A still works well with output

capacitor of other types due to the wide stable ESR

range. Fig.1 shows the curves of allowable ESR range

as a function of load current for various output

voltages and capacitor values. Output capacitor of

larger capacitance can reduce noise and improve

load-transient response, stability, and PSRR. The

output capacitor should be located not more than

0.5" from the VOUT pin of the RT9168/A and returned

to a clean analog ground.

Note that some ceramic dielectrics exhibit large

capacitance and ESR variation with temperature. It

may be necessary to use 2.2

µF or more to ensure

stability at temperatures below -10°C in this case. Also,

tantalum capacitors, 2.2

µF or more may be needed to

maintain capacitance and ESR in the stable region for

strict application environment.

Tantalum capacitors maybe suffer failure due to surge

current when it is connected to a low-impedance

source of power (like a battery or very large capacitor).

If a tantalum capacitor is used at the input, it must be

guaranteed to have a surge current rating sufficient for

the application by the manufacture.

Load-Transient Considerations

The RT9168/A load-transient response graphs (see

Typical

Operating

Characteristics)

show

two

components of the output response: a DC shift from

the output impedance due to the load current

change, and the transient response. The DC shift is

quite small due to the excellent load regulation of the

IC. Typical output voltage transient spike for a step

change in the load current from 0mA to 50mA is tens

mV, depending on the ESR of the output capacitor.

Increasing

the

output

capacitor’s

value

and

decreasing the ESR attenuates the overshoot.

Shutdown Input Operation

The RT9168/A is shutdown by pulling the SHDN

input low, and turned on by driving the input high. If

this feature is not to be used, the SHDN input should

be tied to VIN to keep the regulator on at all times

(the SHDN input must not be left floating).

Fig. 1

COUT = 4.7

µF

COUT = 1

µF

Region of Stable COUT ESR vs. Load Current

120

160

200

Load Current (mA)

100

0.001

0.01

0.1

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